The present invention relates to a nonvolatile semiconductor memory device and a method for fabricating the device, and more particularly relates to measures to be taken to improve the reliability of a nonvolatile semiconductor memory device.
A known nonvolatile semiconductor memory device, like a flash EEPROM, typically has a structure in which a floating gate electrode for storing charge thereon is interposed between a control gate electrode, functioning as a gate electrode for an MIS transistor, and a channel region. Normally, information stored on the floating gate electrode can be read by determining the state of the MIS transistor as ON or OFF, which depends on whether there is any charge stored on the floating gate electrode or not. In a nonvolatile semiconductor memory device like this, information on the floating gate electrode is alterable by utilizing a charge tunneling phenomenon occurring in a tunnel insulating film located under the floating gate electrode. That is to say, the charge can be injected into, or removed from, the floating gate electrode by taking advantage of that tunneling phenomenon. A tunnel insulating film is usually an oxide film. However, it is already known empirically that the tunnel insulating film as a gate insulating film deteriorates with time, or through repetitive passage of charge through the tunnel insulating film. For that reason, various techniques have been proposed to improve the reliability of the tunnel insulating film.
Hereinafter, it will be described with reference to FIGS. 13A through 13D how the tunnel insulating film can have its reliability improved in a known nonvolatile semiconductor memory device.
First, in the process step shown in FIG. 13A, an isolation region 102 and an active region 103, surrounded by the isolation region 102, are defined in a p-type semiconductor substrate 101. Next, in the process step shown in FIG. 13B, a tunnel insulating film 104a is formed to a thickness of about 10 nm on the surface of the substrate 101. Then, in the process step shown in FIG. 13C, first polysilicon, ONO and second polysilicon films are stacked in this order over the substrate, and then patterned, along with the tunnel insulating film 104a, into a predetermined shape. In this manner, a gate electrode section 108, including floating gate electrode 105, interelectrode insulating film 106 and control gate electrode 107, is formed. As used herein, the xe2x80x9cONO filmxe2x80x9d is a multilayer structure consisting of oxide, nitride and oxide films that have been stacked one upon the other. Finally, in the process step shown in FIG. 13D, a sidewall 109 is formed on the side faces of the tunnel insulating film 104, floating gate electrode 105, interelectrode insulating film 106 and control gate electrode 107. Then, ions of an n-type dopant are implanted into the substrate 101 using the gate electrode section 108 and sidewall 109 as a mask, thereby defining n-type source/drain regions 110 and 111 in the substrate 101 on the right- and left-hand sides of the gate electrode section 108.
In the prior art, a write operation is performed by injecting electrons from the channel region, which is part of the substrate 101 located under the tunnel insulating film 104, into the floating gate electrode 105 by way of the tunnel insulating film 104. The electrons may be injected by utilizing an FN tunneling phenomenon, for example. On the other hand, the electrons can be removed from the floating gate electrode 105 into the channel region of the substrate 101. However, it is known that the greater the number of times the electrons pass through the tunnel insulating film 104 by the FN tunneling, the greater the number of defects (e.g., trap sites) created in the tunnel insulating film 104 and the less reliable the film 104 becomes. Thus, a proposed technique attempts to suppress the creation of defects such as trap sites by diffusing nitrogen atoms into the oxide film as the tunnel insulating film.
However, I found as a result of various experiments that even if nitrogen atoms are diffused into the tunnel insulating film, it is still difficult to suppress the degradation of the tunnel insulating film effectively. So I looked into the reasons to make the following findings.
Generally speaking, when nitrogen atoms are diffused into a silicon dioxide film (which is a thermal oxide film), the lower part of the tunnel insulating film closer to the substrate has its quality improved, whereas the upper part thereof closer to the floating gate electrode does not. This is because the nitrogen atoms exist at a relatively high density in the lower part of the tunnel insulating film near the interface with the substrate, while almost no nitrogen atoms exist in the upper part of the tunnel insulating film near the interface with the floating gate electrode.
In a known annealing process for forming a thermal oxide film, the oxide film is usually formed by a pyrolytic oxidation using oxygen and hydrogen gases. In an oxide film formed by the pyrolytic oxidation, a lot of oxygen atoms are contained. However, it is known that these oxygen atoms terminate dangling bonds included in the oxide film, thereby reducing a stress produced in the underlying semiconductor substrate and contributing to the performance enhancement of the resultant transistor. That is to say, it is known that a silicon dioxide film formed by the pyrolytic oxidation is much more reliable than a counterpart formed by a dry oxidation using oxygen gas only.
However, the experimental data I collected told me that while nitrogen atoms were being diffused into a thermal oxide film, hydrogen atoms, which had been introduced into the thermal oxide film by a pyrolytic oxidation process, might diffuse outward. This experimental data will be detailed later. And I believe that a tunnel insulating film, which has been subjected to the nitrogen diffusion process, has its quality degraded because hydrogen atoms, existing near the surface of the oxide film, diffuse outward to create charge trapping sites near the surface as a result of an annealing process at an elevated temperature. Hereinafter, it will be described how I believe the tunnel insulating film deteriorates.
FIG. 14 is a band diagram illustrating energy band structures for a cross section passing the floating gate electrode, tunnel insulating film and semiconductor substrate. Specifically, FIG. 14 illustrates how electrons are injected from the substrate into the floating gate electrode by way of the tunnel insulating film. As shown in FIG. 14, while electrons are injected from the substrate into the floating gate electrode by utilizing the FN tunneling, dangling bonds may exist in the upper part of the tunnel insulating film (i.e., a thermal oxide film where nitrogen atoms have been diffused) near the floating gate electrode. This is because hydrogen atoms may have diffused outward and may be absent from that part. In that case, holes may be trapped at those dangling bonds. In the lower part of the tunnel insulating film near the substrate on the other hand, nitrogen atoms exist at a relatively high density as described above. Accordingly, it is believed that even if dangling bonds have been formed there due to the outward diffusion of hydrogen atoms, those dangling bonds are terminated with the nitrogen atoms and the probability of hole trapping is not so high there.
FIG. 15 is a band diagram illustrating energy band structures for a cross section passing the floating gate electrode, tunnel insulating film and semiconductor substrate. Specifically, FIG. 15 illustrates a state where holes have been trapped in the upper part of the tunnel insulating film near the floating gate electrode. As shown in FIG. 15, if holes have been trapped in the upper part of the tunnel insulating film, then the energy band structure of the tunnel insulating film changes so that the potential level locally drops in part of the conduction band of the tunnel insulating film. As a result, the charges (or electrons) stored on the floating gate electrode may easily leak out of the electrode into the substrate due to the tunneling phenomenon to decrease the reliability of the memory cell. For example, the data stored on the electrode might be lost partially. It should be noted that although some electrons are also trapped at the dangling bonds, but many other electrons widely exist all over the tunnel insulating film.
A general object of the present invention is clarifying the reason why the tunnel insulating film, or a thermal oxide film where nitrogen atoms have been diffused, has its reliability degraded and thereby providing means for suppressing such degradation effectively.
A more specific object of the invention is providing an improved nonvolatile semiconductor memory device as a memory cell with increased reliability by minimizing the decrease in charge retention capability of the floating gate electrode based on that finding.
Another object of the present invention is providing a method for fabricating the improved nonvolatile semiconductor memory device.
An inventive nonvolatile semiconductor memory device includes: a substrate including a semiconductor region; a tunnel insulating film, which is formed on the semiconductor region out of a silicon dioxide film containing nitrogen; a floating gate electrode formed on the tunnel insulating film; a control gate electrode capacitively coupled to the floating gate electrode; an interelectrode insulating film interposed between the floating and control gate electrodes; and two doped regions defined in the semiconductor region on right- and left-hand sides of the floating gate electrode. This device is constructed so that electrons are injected from a region, which is located near a boundary between one of the two doped regions and part of the semiconductor region under the tunnel insulating film, into the floating gate electrode after having been tunneled through the tunnel insulating film.
According to the present invention, electrons are injected into the floating gate electrode through a particular part of the tunnel insulating film. Thus, even if holes have been trapped at charge trapping sites in that local region, the quantity of charge leaking out of the floating gate electrode hardly increases. As a result, should those charge trapping sites have been newly created due to the outward diffusion of hydrogen atoms in the nitrogen-containing silicon dioxide film, the decrease in the charge retention capability of the floating gate electrode can be minimized.
In one embodiment of the present invention, the semiconductor region may be an n-type semiconductor region, and the two doped regions may be p-type doped regions. In that case, the device is preferably so constructed that the electrons are injected into the floating gate electrode as hot electrons that have been created by a band-to-band tunneling current flowing from the n-type semiconductor region into one of the p-type doped regions. Then, the method of injecting hot electrons created by a band-to-band tunneling current can be made full use of. Specifically, threshold voltages for write and erase operations can be greatly different, data is alterable a greater number of times and writing can be performed in a short time. In addition, should charge trapping sites have been newly created, the decrease in the charge retention capability of the floating gate electrode can be minimized.
In an alternative embodiment, the semiconductor region may be a p-type semiconductor region, and the two doped regions may be n-type doped regions. In that case, the device is preferably so constructed that the electrons are injected into the floating gate electrode as channel hot electrons. In such an embodiment, the channel hot electron injection method can be made full use of. Specifically, a control circuit can be simplified, for example. In addition, should charge trapping sites have been newly created, the decrease in the charge retention capability of the floating gate electrode can be minimized.
A first inventive method for fabricating a nonvolatile semiconductor memory device includes the steps of: a) forming a silicon dioxide film on an n-type semiconductor region by performing an annealing process within an ambient containing oxygen and hydrogen, the n-type semiconductor region being located in a substrate; b) diffusing nitrogen into the silicon dioxide film by annealing the silicon dioxide film within an ambient containing nitrogen; c) forming a memory gate electrode section, which includes a floating gate electrode, an interelectrode insulating film and a control gate electrode, on the silicon dioxide film after the step b) has been performed; and d) defining two p-type doped regions by introducing a p-type dopant into two parts of the n-type semiconductor region that are located on right- and left-hand sides of the floating gate electrode.
According to the first inventive method, while nitrogen is being diffused in the step b), hydrogen is released out of the silicon dioxide film. As a result, charge trapping sites are newly created in the silicon dioxide film, which functions as a tunnel insulating film under the floating gate electrode. However, by using the hot electrons that have been created by a band-to-band tunneling current flowing from the n-type semiconductor region into one of the p-type doped regions, the electrons can be injected through a particular part of the tunnel insulating film into the floating gate electrode. Accordingly, a nonvolatile semiconductor memory device, in which threshold voltages for write and erase operations can be greatly different and data is alterable a greater number of times and can be written in a short time and which includes a floating gate electrode with increased charge retention capability, can be obtained.
A second inventive method for fabricating a nonvolatile semiconductor memory device includes the steps of: a) forming a silicon dioxide film on a p-type semiconductor region by performing an annealing process within an ambient containing oxygen and hydrogen, the p-type semiconductor region being located in a substrate; b) diffusing nitrogen into the silicon dioxide film by annealing the silicon dioxide film within an ambient containing nitrogen; c) forming a memory gate electrode section, which includes a floating gate electrode, an interelectrode insulating film and a control gate electrode, on the silicon dioxide film after the step b) has been performed; and d) defining two n-type doped regions by introducing an n-type dopant into two parts of the p-type semiconductor region that are located on right- and left-hand sides of the floating gate electrode. A dopant profile in a transition region between the p-type semiconductor region and one of the n-type doped regions is different from a dopant profile in a transition region between the p-type semiconductor region and the other n-type doped region.
According to the second inventive method, while nitrogen is being diffused in the step b), hydrogen is released out of the silicon dioxide film. As a result, charge trapping sites are newly created in the silicon dioxide film, which functions as a tunnel insulating film under the floating gate electrode. However, according to this method, channel hot electrons, which are created in a boundary region near one of the two n-type doped regions that has the steeper dopant profile when a channel current flows between the two n-type regions, are used. And by using these channel hot electrons, the electrons can be injected through a particular part of the tunnel insulating film into the floating gate electrode. Accordingly, a nonvolatile semiconductor memory device, in which the control circuit can be simplified and which includes a floating gate electrode with increased charge retention capability, can be obtained.
A third inventive method for fabricating a nonvolatile semiconductor memory device includes the steps of: a) forming a silicon dioxide film on a semiconductor region by performing an annealing process within an ambient containing oxygen and hydrogen, the semiconductor region being located in a substrate; b) diffusing nitrogen into the silicon dioxide film by annealing the silicon dioxide film at a temperature between 800xc2x0 C. and 950xc2x0 C. within an ambient containing nitrogen; c) forming a memory gate electrode section, which includes a floating gate electrode, an interelectrode insulating film and a control gate electrode, on the silicon dioxide film after the step b) has been performed; and d) defining two doped regions by introducing a dopant into two parts of the semiconductor region that are located on right- and left-hand sides of the floating gate electrode. The doped regions have a conductivity type opposite to that of the semiconductor region.
According to the third inventive method, while nitrogen is being diffused in the step b), the release of hydrogen from the silicon dioxide film is suppressible. Thus, the creation of new charge trapping sites can be minimized and the silicon dioxide film can function as a tunnel insulating film under the floating gate electrode. As a result, a nonvolatile semiconductor memory device, including a floating gate electrode with increased charge retention capability, can be obtained.
A fourth inventive method for fabricating a nonvolatile semiconductor memory device includes the steps of: a) forming a silicon dioxide film on a semiconductor region by performing an annealing process within an ambient containing oxygen and hydrogen, the semiconductor region being located in a substrate; b) diffusing at least one of hydrogen and fluorine into the silicon dioxide film by annealing the silicon dioxide film within an ambient containing hydrogen and/or fluorine; c) forming a memory gate electrode section, which includes a floating gate electrode, an interelectrode insulating film and a control gate electrode, on the silicon dioxide film after the step b) has been performed; and d) defining two doped regions by introducing a dopant into two parts of the semiconductor region that are located on right- and left-hand sides of the floating gate electrode. The doped regions have a conductivity type opposite to that of the semiconductor region.
According to the fourth inventive method, hydrogen and/or fluorine is/are diffused in the step b). Thus, the number of charge trapping sites in the silicon dioxide film can be reduced and the silicon dioxide film can function as a tunnel insulating film under the floating gate electrode. As a result, a nonvolatile semiconductor memory device, in which a decreased quantity of charge leaks out of the tunnel insulating film and which includes a floating gate electrode with increased charge retention capability, can be obtained.
In one embodiment of the present invention, the fourth method may further include the step of diffusing nitrogen into the silicon dioxide film by annealing the silicon dioxide film within an ambient containing nitrogen between the steps a) and c). In such an embodiment, the silicon dioxide film, into which nitrogen has been diffused, functions as a tunnel insulating film under the floating gate electrode. Accordingly, a highly reliable tunnel insulating film, in which a much smaller number of defects are created even if electrons have been removed from the floating gate electrode many times, can be obtained. As a result, a nonvolatile semiconductor memory device, including a much more reliable tunnel insulating film, can be obtained.
In another embodiment, the silicon dioxide film is preferably annealed in the step b) at a temperature between 300xc2x0 C. and 950xc2x0 C.
A fifth inventive method for fabricating a nonvolatile semiconductor memory device includes the steps of: a) forming a silicon dioxide film on a semiconductor region by performing an annealing process within an ambient containing oxygen and hydrogen, the semiconductor region being located in a substrate; b) diffusing nitrogen into the vicinity of an interface between the silicon dioxide film and the substrate by annealing the silicon dioxide film at a temperature between 800xc2x0 C. and 1200xc2x0 C. within an ambient containing nitrogen; c) diffusing nitrogen into a surface region of the silicon dioxide film by annealing the silicon dioxide film at a temperature between 300xc2x0 C. and 800xc2x0 C. within an ambient containing nitrogen radicals; d) forming a memory gate electrode section, which includes a floating gate electrode, an interelectrode insulating film and a control gate electrode, on the silicon dioxide film after the step c) has been performed; and e) defining two doped regions by introducing a dopant into two parts of the semiconductor region that are located on right- and left-hand sides of the floating gate electrode. The doped regions have a conductivity type opposite to that of the semiconductor region.
According to the fifth inventive method, nitrogen is diffused into not only around the interface between the silicon dioxide film and substrate but also the surface region of the silicon dioxide film. Thus, the number of holes trapped in the surface region of the silicon dioxide film can be reduced drastically, and the silicon dioxide film can function as a tunnel insulating film under the floating gate electrode. As a result, a nonvolatile semiconductor memory device, in which an even smaller quantity of charge leaks out of the silicon dioxide film as the tunnel insulating film and which includes a much more reliable tunnel insulating film, can be obtained.